I am able to update most of my document's metadata through the REST API with two exceptions:

#1: Date Formats
I can't seem to update any of the attribute fields that are defined with the "Date" type. I've tried a bunch of different formats without luck, but each results in a 500 Intneral Server Error.

This command works fine if the modDate attribute is a String type. It only fails when it is a Date type.

#2: Updating standard properties
I am using the REST API (in part) to migrate documents from our old document management system. I'd like to preserve the created/published date and user. Is there any way to update those fields via the REST API? The following has no effect:

I checked the code and I can confidently say that it is useless to try to change the document's dates, these changes currently are not covered.
In particular the changes to the fields creation, date and lastModified are not taken into account.
Note: I've tested the code against LD 7.6.3

You can only change the date fields startPublishing and stopPublishing